Vision and hearing are generally regarded as two very different senses. Unless, of course, you can echolocate. Now, scientists have revealed for the first time that human echolocators — blind individuals who navigate their surroundings by producing mouth clicks and listening to the returning echoes — actually process these sounds in the regions of the brain dedicated to interpreting visual stimuli.

Almost all of us are familiar with the concept of echolocation, the biological sonar used by animals like bats, dolphins, and whales to locate objects and navigate their environments.

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However, as the video here demonstrates, talented human echolocators like Daniel Kish have also been documented. Using this natural form of echolocation, which is performed without the use of any peripheral aids, human echolocators have been known to participate in activities ranging from basketball to mountain biking.

Documentation of human echolocation goes back hundreds of years, and research dates back to the 1950's, but understanding the extent of this fascinating ability has, until now, been limited to behavioral studies. In their recent publication, however, Canadian researchers Lore Thaler, Stephen Arnott, and Melvyn Goodale have shed new light on the science of human echolocation by investigating which brain areas mediate natural human echolocation.

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To map the neural architecture underlying human echolocation, the researchers placed tiny microphones in the ears of two blind echolocation experts. The first subject has been blind since 13 months of age, and is referred to as the early blind (EB) test participant. The other, who has been blind since the age of 14, is referred to as the late blind (LB) test participant. The researchers recorded their clicks and returning echoes as the echolocation experts tried to identify objects in the environment. The recordings were played back to the subjects and their brain activity was studied with functional magnetic resonance imaging (fMRI), a form of neuroimaging that maps neural activity in different regions of the brain by measuring changes in blood flow to those regions. Two non-echolocating, sighted test participants (C1 and C2) were run as sex and age-matched fMRI conrols for EB and LB, respectively.

The image on the left and the image below are some of the functional MRI results for the EB, LB, and control participants in the study. The images represent the cortical surface (the outermost layer) of each test subject's brain. The activation of specific regions of the subjects' cortices in response to stimuli is indicated by the warm colors ranging in hue from red to yellow.

If you take a look at the image on the left, you'll see the regions of the brain that are activated when the test subjects listen to recordings of echolocation clicks and echoes in a silent environment (in other words, the clicks and echoes are the only auditory stimulus the test subjects are receiving). Go ahead and click on the thumbnail for a high-res image.

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What you're looking for in this figure is the region of the brain labeled "CaS," for calcarine sulcus. This is the region of the cortex typically responsible for processing visual stimuli. You'll notice that in both blind participants – and in the EB patient, especially – there is a significant level of neural activity in this region. (The strength of the signal from the EB participant's visual cortex could reflect EB's much longer use of echolocation and/or his more reliable performance in passive echolocation tasks.) You'll also notice that in the brains of the control participants these regions are devoid of any activity whatsoever.

You may have noticed that, in the above image, all four subjects also demonstrated neural activity in the region of the brain called the lateral sulcus, labeled "LS." This part of the brain is typically responsible for processing auditory stimuli. Its activation in the test participants likely reflects the auditory nature of the stimuli in an otherwise noise-free environment. But in the real world, human echolocators aren't navigating silent environments; they're mountain biking outside, playing basketball at busy public parks, and traversing noisy city streets. So what do these brains look like under more realistic environmental conditions?

Amazingly, when the researchers analyzed the brain's response to clicks and echoes under more realistic, "outdoor" conditions, neural activity disappeared in the auditory cortices of the EB and LB echolocators, but remained in their visual cortices; the brain areas that process auditory information were no more activated by sound recordings of outdoor scenes containing echoes than they were by sound recordings of outdoor scenes with the echoes removed.

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In the image on the left, you'll notice that in the brains of the blind echolocators, the auditory region (circled in pink) of the brain is dark, while the visual region (circled in white) remains lit. In other words, against the background noise of an outdoor environment, the clicks and echoes heard by blind echolocators were processed in the part of the brain that, in sighted individuals, are dedicated to interpreting waves of light, not sound.

The researchers explain:

The data show that the presence of echoes within a train of complex sounds increases [the fMRI signal] in calcarine cortex in both EB and LB. This increase in activity in calcarine cortex is absent in C1 and C2. Importantly, the presence of echoes within a train of complex sounds does not lead to an increase in BOLD signal in auditory cortex in any of the four participants. This finding suggests that brain structures that process visual information in sighted people process echo information in blind echolocation experts.